1. Field of the Invention
This invention relates to a method of manufacturing a semiconductor device for depositing a film being composed of silicon and germanium, and a semiconductor manufacture system for practicing the method of manufacturing the semiconductor device.
2. Description of Related Art
Hetero Bipolar Transistor (HBT) using SiGe or SiGeC attracts a great deal of attention because of its high-speed, low-noise, and low electricity. FIG. 3 is a diagrammatic view which shows a composition of HBT using SiGe (SiGe-HBT). As shown in this drawing, SiGe-HBT is provided, in its base region, to include Ge which has smaller band gap than Si (silicon) and is added therein subsequently within the range of 0-30% to have a slope on its conduction band. This presents a drift electric field and accelerates electron to thereby attain the high speed.
To be more precise, SiGe-HBT typically utilizes a silicon wafer as silicon substrate thereof to obtain a SiGe film as its base layer by epitaxially growing SiGe mixed crystal on the silicon wafer. The base layer is so composed that a concentration of Ge gets smaller composition ratio of Ge from a collector layer to an emitter layer in a film thickness direction thereof. HBT using SiGeC (SiGeC-HBT) can be naturally composed as those in SiGe-HBT.
When SiGe or SiGeC (hereinafter referred to as SiGe (C)) as the base layer is deposited on the collector layer in order to manufacture SiGe-HBT or SiGeC-HBT and other film, a Ge fraction in the film needs to be decreased in sequence or in phase.
When SiH4 (monosilane) and GeH4 (germane) are used as reactive gases (source gases), and H2 (hydrogen gas) is used as a carrier gas, the Ge fraction in the SiGe(C) film can be controlled by a gas partial pressure rate of SiH4 and GeH4 i.e., a flow rate in a reaction chamber where the silicon wafer is placed. A typical example will be described as follows.
FIG. 4 shows a partial pressure dependence of GeH4 regarding Ge fraction when depositing SiGe. In FIG. 4, a horizontal axis indicates GeH4 partial pressure (proportional to a flow rate of GeH4), and a vertical axis indicates Ge fraction in the SiGe film, when fixed at 500° C. as deposition temperature, at 30 [Pa] as total pressure in the reaction chamber, and at 6 [Pa] as SiH4 partial pressure. As can be seen from this drawing, by decreasing the GeH4 partial pressure, a region of small Ge fraction can be obtained. This means Ge fraction can be smaller by decreasing flow rate of GeH4.
FIG. 5 shows a partial pressure dependence of GeH4 regarding deposition rate when depositing SiGe. In FIG. 5, a horizontal axis indicates GeH4 partial pressure (a flow rate of GeH4), and a vertical axis indicates deposition rate [nm/min] in the SiGe film, when fixed at 500° C. as deposition temperature, at 30 [Pa] as total pressure in the reaction chamber, and at 6 [Pa] as SiH4 partial pressure. Common logarithm is used on the vertical axis in FIG. 5. As shown in this drawing, the smaller GeH4 partial pressure is, the slower SiGe deposition rate is.
FIG. 4 and FIG. 5 show the SiGe deposition state as one typical example, and the same results will be available when SiGeC is used.
Accordingly, when SiGeC is deposited under the above-mentioned condition, the region of the smaller Ge fraction can be formed by sequentially decreasing the GeH4 flow rate. However, SiGeC deposition rate will be slow by decreasing a flow rate of GeH4. In other words, forming time for a region of low Ge fraction is longer than that for a region of high Ge fraction. The details will be verified hereinafter.
As an experimental example of a conventional invention, a SiGe film is formed under the condition at 500° C. as deposition temperature, at 30 [Pa] as total pressure in the reaction chamber, and at 6 [Pa] as SiH4 partial pressure. While changing Ge fraction by negative one percent from 25% to 1%, the SiGe films each containing a relative Ge fraction were made, each being 1.5 nm thick and the total of them being 37.5(=25×1.5) nm thick.
FIG. 6 shows the relationship between GeH4 flow rate, Ge fraction and deposition time. In FIG. 6, a horizontal axis indicates GeH4 flow rate [sccm], and both vertical axes indicate Ge fraction [%] and deposition time [min], respectively.
In FIG. 6, a continuous line plotted with white dots shows change of Ge fraction in relation to GeH4 flow rate. As shown in this drawing, these two are almost proportional under this condition. Ge fraction varies by the rate of negative one percent from 25% to 1%.
In FIG. 6, a bar graph is also drawn to show the time to make the SiGe film of 1.5 nm thick every plot at the corresponding flow rate of GeH4 (Ge fraction).
As shown in FIG. 6, in case of obtaining the SiGe film with the same thickness (1.5 [nm]), the smaller the GeH4 flow rate is, and that means the lower the Ge fraction is, the more it takes time for deposition. As has been described above, this is because the smaller GeH4 flow rate is, the slower deposition rate is. For example, when the flow rate of GeH4 is 50 [sccm], the necessary time to deposit a film of 1.5 nm thick is 1.3 minutes. In contrast, when the flow rate of GeH4 is 10 [sccm] (Ge fraction is 6%), the necessary time to make its counterpart is 4.3 minutes.
In this case, the total time to obtain the SiGe film of 37.5 nm thick was 70 minutes.
As described above, there was a problem that deposition rate would be slow and deposition time would be long as the GeH4 flow rate would be low in the case of decreasing Ge ratio sequentially on making a SiGe (C) film.
In order to solve this problem, patent application Number: Hei 7-297205 proposes, by using the so-called cold wall type single-wafer CVD system, a manufacturing method of a semiconductor device for decreasing a GeH4 flow rate while rising temperature between 5 to 30° C./minute. According to this method, by making a SiGe film while rising temperature by the above-mentioned degrees, deposition rate will not be slow when forming a region of small Ge ratio. Therefore, deposition time will be shorter than that when keeping the same deposition temperature.
The above-mentioned technique demonstrated faster deposition rate than conventional techniques for deposition; however, many issues still remain being unsolved about a request of the field. Especially, there is a room for improvement about how to make even thickness of the SiGe(C) film.
To be more precise, while changing deposition temperature to obtain the film by using this conventional method, thermal distribution of wafer becomes uneven while making it. Therefore, the thickness of the SiGe (C) film tends to be uneven.
This problem becomes especially serious when using a hot wall batch-type system which produces large quantity of films. That is, heat conduction of the inner side of the wafer is slow compared with that of the outer side. Therefore, when making the SiGe (C) film by rising deposition temperature, the outer side of the wafer has higher temperature than the inner side. The film thickness of the outer side becomes thick, and that of the inner side becomes thin.